1. Field of the Invention
This invention relates to a tool changing method and, more particularly, to a tool changing method in a numerical control unit wherein a tool used in a following process is held in standby at a standby position in concurrence with machining of a present process.
2. Description of the Related Art
As shown in FIG. 6, an NC program for controlling a machine tool having an automatic tool changing apparatus (ATC apparatus) is composed of tool change commands ATCi (M06T.sub..quadrature..quadrature..quadrature. ;) used in respective ones of machining processes MP1, MP2, MP3, . . . , and machining commands MCi (i=1, 2, . . . ) for machining performed by the tools according to the prior art. In FIG. 6, "M06" is an M-function instruction for an automatic tool change, and "T .sub..quadrature..quadrature..quadrature. " is a tool selection command for a tool having a tool number indicated by .sub..quadrature..quadrature..quadrature..
However, an NC program with such a configuration cannot be applied to an automatic tool changing apparatus having a function in which a tool used in a following process is held in standby at a standby position in concurrence with machining of a present process. If such an NC program were to be applied, the standby function of the automatic tool changing apparatus would not be capable of fully demonstrating its ability and the tool change would require too much time.
For this reason, in a case where the automatic tool changing apparatus possesses a standby function, the NC program is so arranged that a command PTCi for holding the tool used in the next process at a standby position is inserted ahead of the machining command MCi in the present machining process MPi, and the tool for the next process is held in standby at the standby position in concurrence with the machining of the present process. In accordance with this NC program, the tool used in the next process can be held in standby at the standby position during the machining performed in the present process. Accordingly, when the machining of the present process ends, machining can be performed immediately using the tool held in standby. This is advantageous in that the apparent time required for the tool change can be made zero, as a result of which machining efficiency can be improved.
In such an NC program configuration, however, the programmer must carry out programming while constantly being aware of the tool used in the next process. As a result, programming is troublesome and prone to error. In addition, once the NC program has been created, replacement of a process in the program cannot be performed merely by making the replacement in process units, as in an NC program having the conventional configuration illustrated in FIG. 6. Since the command portion for the tool used in the process must also be changed, program modification processing becomes a troublesome task.